When firing semi-automatic and automatic firearms in law enforcement, military actions and in target shooting competition it is desirable to know when the magazine is about to run out of ammunition. Competition shooters need to know this information so they can be prepared to release the empty magazine and replace it with a full one with minimum loss of shooting time. In police and military operations the need to know is far more serious. Police officers and soldiers can lose their lives in the split second it takes to realize a magazine must be replaced, or an enemy or felon can escape in that time. And studies have shown that under the stress of a firefight it is nearly impossible for the user to keep accurate track of the cartridges fired and those remaining. Prior attempts to monitor the number of cartridges in a magazine by indicating the number expended or the number remaining have met with indifferent success, and by and large applied generally only to pistols and not other firearms. The apparatus often was large and cumbersome and required modifications to each firearm for retrofitting or required redesign for installation with original equipment manufacturers. Such systems did not distinguish between an empty condition and a jammed condition. There were often on-off switches: a real drawback when the user forgets to turn on the system. The prior art designs often relied solely on displaying a count of the number of cartridges spent or remaining, which meant that the shooter had constantly to be watching the display: there was no alarm that communicated urgency. And the brightness of the display was fixed and not adaptable to ambient light conditions or shooters with poorer eyesight. Often the display used an LCD which is not visible in dark or low light conditions, a serious problem in many law enforcement situations. Typically there was no way to test the battery or other power supply or the display to see that the monitoring system was working. The known systems also impeded the free-fall of a released magazine and were not waterproof, a serious shortcoming in police and military applications. And the addition of the monitoring system to the firearm often interfered seriously with the critical ergonomics of the firearm.
In one prior art system, disclosed in U.S. Pat. No. 5,303,495 to Harthcock, only the number of cartridges in the magazine of the weapon are taken into account. As shown in FIG. 1 of that patent there are eight Hall effect switches corresponding to eight cartridges within the magazine. As a spring moves up within the magazine when each cartridge is loaded into the firing chamber a magnet on the spring is moved into proximity with a different switch. The magnet activates the switch and the system provides an indication of the number of cartridges remaining in the magazine. However, this system does not account for the cartridge in the firing chamber. Therefore, any count displayed by that system is ambiguous. It could indicate that the number of cartridges in the magazine is the total number of cartridges in the firearm, but only when no cartridge is actually in the firing chamber. With the same reading, there could be an additional cartridge in the firing chamber not indicated on the display. The only way the user will know for sure is to check the firing chamber. This is obviously unacceptable under most circumstances, such as in a firefight.
In another prior art system, disclosed in U.S. Pat. No. 5,052,138 to Crain, the follower in the magazine engages contact strips on the interior walls of the magazine to provide a count of the number of cartridges in the magazine. A separate external sensor to monitor the reciprocal movement of the slide of the weapon to account for the cartridge in the chamber must be provided. That system is mechanically complex and requires significant modification of an existing weapon or the user must purchase a weapon manufactured to include the separate external sensor. Moreover, the weapon must also be equipped with complicated circuitry to combine the signal from the external sensor and the signal from the contact strips to indicate the cartridge count in the weapon.
In yet another prior art system, disclosed in U.S. Pat. No. 5,142,805 to Horne et al., when a magazine, ordinarily containing for example eight cartridges, is inserted into the firearm, a microcontroller sets the number of cartridges to eight and displays this number on an LCD. When the firearm slide moves rearwardly upon firing of the firearm or upon manual retraction of the slide, the cartridge count is decremented by one. This system therefore, only operates properly when a fully loaded magazine with a predetermined number of cartridges is loaded into the firearm. This system cannot give an accurate count of cartridges in the firearm when a partially depleted magazine is inserted unless the microcontroller is manually re-programmed with the number of cartridges in the partially depleted magazine. This system is therefore not very useful, especially given the fact that it is standard combat operating procedure to eject a partially depleted magazine to ensure that the firearm never becomes unexpectedly empty and then to later reload the partially depleted magazine. Moreover, if, due to human error, a magazine which is not fully loaded is inserted into the firearm the Horne et al. system will not provide an accurate cartridge count.